1. Field of the Invention
The present invention relates to a protection circuit of a power supply.
2. Description of the Prior Art
Switching mode power supplies (SMPS), which control on and off states of a power switch to store and release energy in an inducting unit to provide required power to a load, make up the majority of power supplies due to conversion efficiency and product size thereof.
For example, FIG. 1 is a diagram showing a SMPS 10 with a flyback structure. A bridge rectifier 12 rectifies AC power to provide DC power VIN, which may be as high as 100 Volts to 300 Volts, at an IN end. A controller 18 detects a detection signal VCS across a current sensing resistor 16 via a CS end. The detection signal VCS is a voltage signal corresponding to an induction current through a primary winding 24 of a transformer 20 when a power switch 15 is turned on. The controller 18 increases or decreases the induction current by controlling on and off states of the power switch 15 via a GATE end. A secondary winding 22 provides output power VOUT to a load 30. An auxiliary winding 23 provides operating power VCC to the controller 18.
Most switching mode power supplies need a protection mechanism to prevent abnormal conditions. A common protection mechanism is called over voltage protection, which turns off the power switch for a period of time when the output voltage VOUT is too high.
FIG. 2 is a diagram showing an over voltage protection mechanism. In FIG. 2, an over voltage protection circuit is arranged in the controller 18. If a comparator 32 determines a voltage of the operating power VCC exceeds a reference voltage VREF1, the over voltage protection is triggered. However, the voltage of the operating power VCC may not correctly correspond to the output power voltage VOUT due to inductor leakage. Therefore, the protection mechanism of FIG. 2 is not proper.
FIG. 3 is a diagram showing another over voltage protection mechanism. When the output power voltage VCC exceeds a predetermined voltage of a Zener diode 38, a photo-coupler 36 pulls down a voltage at an input end of a comparator 34 to trigger an over voltage signal SOVP. However, the protection mechanism of FIG. 3 needs the additional Zener diode 38 and photo-coupler 36, which increases cost and product size.